In part 2 peripheral lymphocyte count, the total number of spleen cells, the weight and histology of the spleen, in vitro proliferative response of spleen cells to phytohemagglutinin (PHA) and delayed cutaneous hypersensitivity to picryl chloride were studied to see the effects of a new antileukemic agent, Neocarzinostatin (NCS), on the cellular immunity of mice. A single administration of NCS decreased the peripheral lymphocyte count, the spleen weight, the total number of spleen cells and in vitro proliferative response to PHA on the 4(th) and 8(th) day after NCS administration but its suppressive effects on these factors almost disappeared on the 12(th) day. NCS suppressed the delayed cutaneous hypersensitivity to picryl chloride and its suppressive effect was dependent on the daily dose of NCS. But its suppressive effect weakened after 10 days of drug free interval. The above data suggest that NCS suppresses the cellular immunity of mice to a certain degree and its immunosuppressive effects are dependent on the daily dose and the number of consecutive days of NCS administration, and on the days of drug free interval.